This invention relates generally to a method of abrading a hardened resin and, more specifically to a method of removing an excess resin layer of an alkali-soluble resin formed on a metal-plated board during the plugging of metal-plated through-holes of the board with the alkali-soluble resin.
Recent development of electronics art demands printed wiring boards having a high circuit density. Thus, printed wiring boards provided with wiring patterns on both sides thereof are now increasingly used.
One known method for the production of such a both-sides wiring boards includes the steps of: drilling or punching a board to form a multiplicity of through-holes, plating copper on both sides of the board and on inside walls of the through-holes, plugging the through-holes with an alkali-soluble resin, curing or hardening the resin within the through-holes, abrading or sanding both surfaces of the metal-plated board to remove a layer or deposit of the resin thereon, screen printing positive patterns with an alkali-soluble resist, curing the resist patterns, etching the exposed metal layer with an acidic etching liquid, and removing the resist patterns and the plugged resin using a 2-3% aqueous sodium hydroxide solution.
The plugging of the through-holes is generally performed as follows. A curable or hardenable resin is first applied to the board by means of, for example, a roll coater to plug the through-holes. An excess resin on the surface of the board is then removed by a squeegee and the resin plugged in the through-holes is heat-treated or UV-treated for effecting curing or hardening. The resulting board is then sanded or abraded by means of a buff roll or a belt sander to remove a layer of the hardened resin, which has been failed to be removed during the treatment with the squeegee, and to clean the surface of the metal-plated board.
As a resin for plugging through-holes of the board, there has been used a heat-curable or a photo-curable resin. Since a photo-curable resin can be hardened within a shorter period of time as compared with a heat-curable resin and since the former resin is substantially free of change of volume upon hardening, recent trend is toward the use of the former resin. The former resin, however, poses a problem that it is difficult to remove hardened resin by abrasion because of its hardness.
In particular, the removal of the hardened resin by abrasion with a belt sander tends to cause abrasion or injury of the metal layer to which the hardened resin sticks so that in the subsequent screen printing step a photoresist ink tends to blur the surface of the metal layer or the ink pattern fails to firmly adhere thereto. Further, the abrasion of the metal layer sometimes causes electrical disconnection. Thus, in order to prevent such abrasion of the metal layer, it is necessary to operate the belt sander slowly and gently. This, however, requires a long process time.